Since no Jake brake exists for my present Cat engine, as an auxillary braking system, I'm considering an Allison MT643R with retarder. I already have an MT643 in my Thomas, but am curious if there are any advantages to any other 600 series Allisons, or if the 643R will do the job. An overdrive model perhaps?According to the Allison site, the R model MT643 only weighs about 50 extra pounds, adds around a quart of tranny fluid and no extra length.

You must have the infamous 3208 Cat. Is it turbo'd? How much driveshaft length do you have? The hydraulic retarder on any of the Allisons was designed primarily for stop and go service in the city. When it come so long down grades, they tend to heat up the transmission. Course an additional aux trans cooler will take care of much of that. I believe that the output retarder on the MT added about 3" to the length of the transmission. Since they still make parts for the MT600 series, I'd say look for one. Also you could add a Telma electric retarder directly to the rear axle. They are heavy and expensive, but are the most powerful retarder available-up to 800hp of braking. With the Telma you should have at least a 200 amp alternator and four starting batteries to cover the large current draw when it is in use. I can tell you from experience of putting the Telma on trucks, they work extremely well. The Allison out put retarders work relatively well, but do have the capability to completely stop the bus since they also installed a multiplate wet brake (think wet clutch in reverse) to continue to stop the vehicle under 10mph. Good Luck, TomC

Clifford,Although an exhaust brake may be available for the 3208, I personally do not consider it an option in this case.

Tom,Thanks for stepping up with some good information, I was hoping to hear from you on this issue.Yes, I'm cursed with the dreaded 250hp Cat 3208 turbo...LOL...So far the only problem it has given me is that stinkin' Cat exhaust. It has started on it's own down to 15*, only used a gallon of oil in 10,000 miles, and doesn't leak...anything. And although I haven't had to try it out, I'm kinda keen on the built in priming handle on the fuel pump.

I thought about a Telma, but as you say, it's a lot more weight and expense. I have a 200 amp Leece Neville alternator, but only room on my coach battery tray for 2 8D's.That's why I'm leaning towards the 643R. Depending on where I find one, and how proud the owner is, I figured I might get into a good one, or some conversion parts, throw in the R&R, largest pan, hardware, and LARGE fluid cooler, then call it good. My present driveshaft is about 15-18 inches long, in need of U joints and a pinion seal, so I guess I should hold off on that repair until I see if I need to shorten the driveshaft to retrofit the R model 643.

HAHAHA I must be living right. Although the Allison MT643R transmission with retarder seems to be extremely rare, I found out that the internal/external retarder parts needed to convert my MT643 to an R model, are the same on a MT640, 643, 653, 644, 647, 654, as well as the HT740, 741, 746, 747, 748, 750, 754, 755, 777, and should be plentiful all over the country.

Thanks for your reply, Tom Y! Let me know if that 748 deal craps out, and maybe we can do some biz. I found an online vendor with all the parts I need, but haven't been able to get a price from them yet. I'd really rather have the entire tranny, so I have all the bolts and fasteners as well.